Multiple daily injections of insulin remain the traditional approach for the treatment of insulin-dependent diabetic patients (Drug Discovery Today 2001, 6: 1056-1061). However, suboptimal control of blood glucose level and poor patient compliance are the associated disadvantages with this treatment (Drug Dev Res 2004, 63: 151-160). Oral insulin delivery is a more convenient way to administer insulin to diabetic patients as it is the most physiologically comfortable means (Pharm Technol 2001, 10: 76-90; Journal of Drug Targeting 2010, 18: 79-92). Nevertheless, creating an oral formulation is a daunting task for all bioactive macromolecules, due to the highly organized barriers that the macromolecules encounter in the gastrointestinal (GI) tract, such as rapid enzymatic degradation and the poor intestinal absorption (J Controlled Release 2000, 64: 217-228). In addition, protein drugs that possess narrow absorption windows often exhibit variable exposures leading to their poor drug transportation in the GI tract (J Pharmacol Exp Ther 2001, 297: 638). For many years, various strategies have been developed to enhance oral delivery of insulin (Advanced Drug Delivery Reviews 2007, 59: 1521-1546; Diabetes Obesity & Metabolism 2010, 12: 179-185). Polymeric nanoparticles are of special interest due to the pharmaceutical advantage such as enabling modulation of physicochemical characteristics (Critical reviews in therapeutic drug carrier systems 2005, 22: 419-463). Moreover, their submicron size and their large specific surface area favor their absorption compared to larger carriers (International journal of pharmaceutics 2004, 285: 135-146; Advanced drug delivery reviews 2007, 59: 631-644).
In order to overcome the barriers mentioned above, oral insulin nanoparticles have been widely investigated to increase their biological activity in experimental animals (Expert Opinion on Drug Delivery 2008, 5: 45-68; Biodrugs 2008, 22: 223-237; J Controlled Release 2006, 116: 1-27). The first obstacle for oral delivery of insulin is in the stomach which forms the boundary between the intestine and the external environment. Special pH-sensitivity of nanoparticles provides the protection to confront this first barrier by preventing insulin from contacting the highly acidic medium in the stomach (Amino Acids 2009, 37: 97-97; J Controlled Release 2008, 132: 141-149; J Pharm Sci 2007, 96: 421-427; Journal of Biomedical Materials Research Part B: Applied Biomaterials 2006, 76: 298-305; Eur J Pharm Sci 2007, 30: 392-397; Biomaterials 2009, 30: 2329-2339). The second barrier for oral delivery of insulin is poor intestinal absorption. To overcome this barrier, special mucoadhesive nanoparticles are developed to prolong insulin nanoparticles' intestinal residence time and increase the permeability of mucosal epithelium, thus finally facilitating insulin entering into systemic circulation (Biomaterials 2009, 30: 2329-2339; Biomaterials 2009, 30: 5691-5700; J Pharm Sci 2009, 98: 4818-4830; Biomacromolecules 2008, 9: 278-285; Int J Pharm 2007, 342: 240-249; U.S. Pat. No. 7,871,988 B1). Double-functional nanoparticles with both pH-sensitivity and mucoadhesivity can overcome all the barriers mentioned above (Biomaterials 2010, 31: 6849-6858). Inserting the double-functional nanoparticles into the enteric-coated capsule could protect against the pH instability of nanoparticles in the stomach (Biomaterials 2010, 31: 3384-3394). It has been suggested that the positive charge of insulin nanoparticles is a positive factor for insulin absorption (Int J Pharm 2000, 194: 1-13). The nanoparticles having a more positive charge are more effective on opening tight junctions, leading to an increase in paracellular permeability (Nanotechnology 2007, 18: 1-11). However, a polycationic nanoparticle with mucoadhesivity and pH-sensitivity may be a non-synergistic carrier for insulin, since the positive charge of the polymer in those nanoparticles could reduce the stability of the nanoparticles in the stomach and the pH-sensitivity of the polymer in those nanoparticles could weaken the positive charge of the nanoparticles in the intestine (Biomaterials 2009, 30: 2329-2339).
Fortunately, the GI barriers are sequential in nature; therefore the probability of reaching the therapeutic objective is the contribution of each individual probability to overcome each barrier (Current opinion in chemical biology 2005, 9: 343-346). The multistage delivery system has a separate intended function, which can efficiently overcome various barriers and simultaneous delivery of independent systems (Proceedings of the National Academy of Sciences 2011, 108: 2426; Biochimica et Biophysica Acta 2011; Nature nanotechnology 2008, 3: 151-157; U.S. Pat. Pub. No. 2008/0311182 A1).
Thus, a two-stage delivery system is needed for allowing a high degree of selectivity in the stage 1 enteric capsule and in the stage 2 cationic nanoparticles Such a two-stage deliver system would have an excellent synergistic effect together with pH-sensitivity and a mucoadhesive property.